Interferon has been shown to have a profound effect on tumor growth, viral replication, and the immune response. We have identified an inhibitor of interferon action which is produced by mitogen-stimulated mouse spleen cells. This inhibitor blocks the antiviral and immunoregulatory activity of interferon. This study will employ inhibitor produced and purified in bulk quantities so it may be characterized and employed to study the modulation and control of the antitumor and antiviral activities of interferon. We have also discovered that mixed preparations of mouse immune and fibroblast interferons provide a 10- to more than 20-fold greater level of antitumor and antiviral activity than expected on the basis of the additivity of the two interferons. This tremendous increase in interferon activity (potentiation) has been noted for both in vivo and in vitro antitumor systems as well as in vitro antiviral systems. This study will more carefully quantify the potentiation phenomenon and will seek to elucidate the mechanisms by which this potentiation occurs.